After trial and error with my first two kids, I finally decided that their third birthday was the best time for weaning. By that time they are old enough to understand some basic reasoning tactics. Before that, they are not old enough to understand why you are cutting them off from their favorite passtime. Breastfeeding is so much more than just milk. It is so deeply entrenched in the comfort, security, nutrition and extra health benefits they have known since birth, that weaning can be a very traumatic experience. On the other hand, some kids just one day decide they've had enough and wean themselves, go figure, I never had one do that myself, but I've heard of it happening.
I would start one week before their third birthday, talking to them about how big they were going to be when they turned three. I would entice by promises (real, never idle!) of big boy (or girl) underpants, rides on the big swing, instead of the baby swing, etc. stressing all the things they would be old enough to do, then I'd mention, that as well as the things (s)he was big enough to do, there would also be some things (s)he would be too big to do, such as nursing, pooping their pants, etc. This worked so well, I used it on my last 5 kids!
You could try that tactic earlier, if you want, say make a goal of the 4th of July, or Christmas, or some other big occasion that happens to come in the time frame you're wanting.